In recent years, a variety of projectors have been widely used in various video applications. For example, the projectors can be used for making presentations, holding meetings or giving lectures in classrooms, board rooms, conference rooms or home theaters. By a projector, an image signal from an image signal source can be enlarged and shown on a projection screen.
FIG. 1 shows the light path from the lamp to the projection lens in a digital light processing (DLP) projection system according to a prior art. As shown in FIG. 1, the projection system 1 includes a lamp 11, a light integration rod 12 (i.e. light tunnel), a set of relay optics 13, a total internal reflection prism 14 (TIR prism), a digital micro-mirror device 15 (DMD) and a projection lens 16. The lamp 11 with an elliptical reflector can output light beam 11a and converge the light beam 11a into the light integration rod 12. The uniformed light outputted from the light integration rod 12 is relayed through the set of relay optics 13 and the TIR prism 14 and then outputted to the DMD 15. Then, the uniformed light is modulated by the DMD 15 and been projected onto screen (not shown in FIG. 1) via the projection lens 16.
FIG. 2 shows the convergent light transmitted to the entrance of the light integration rod of FIG. 1. When using the light integration rod 12, the entrance and exiting surfaces are usually coated with anti-reflection (AR) layers to increase transmittance. Because the convergent light beam 11a from the lamp 11 is focused onto the entrance surface 121 of the light integration rod 12, the surface temperature on the entrance surface 121 of the light integration rod 12 is relatively high. So, it is important for the AR coating formed on the entrance surface 121 of the light integration rod 12 to resist high temperature.
As the power of the lamp 11 is increased, the temperature on the entrance surface 121 of the light integration rod 12 is also increased a lot. It is possible for dust to stick on the surface of the integration rod 12 as the projection system 1 ages. The surface temperature where the dust sticks will be increased due to the heat absorption of the dust, such that the AR coating could be damaged. In this situation, the AR coating is usually removed to solve this problem. However, this will degrade luminous flux due to lower light transmittance of the light integration rod 12. In general, the refraction index of a glass is around 1.5. Base on Fresnel Equation, the reflectivity of a glass without AR coating is around 4%. Also, a suitable AR coating in general can reduce the reflectivity of the glass to be lower than 1%. This means the optical system will lose around 3% of light if the AR coating of the light integration rod 12 is removed.
Therefore, there is a need of providing a light integration module and an optical system to obviate the drawbacks encountered from the prior arts.